Pore-forming toxins (PFTs) are the single largest class of bacterial protein toxins known and are employed by many pathogenic bacteria (e.g., Staphylococcus aureus and Streptococcus pyogenes) to cause disease. These toxins punch holes in cell membranes and disrupt normal cell functions. Despite their importance, how PFTs work, how host cells respond to them, and whether or not animal cells have a defense against this form of attack are poorly understood. Recently, the nematode Caenorhabditis elegans has emerged as an excellent system for studying PFTs. Using C. elegans, it has been demonstrated that the p38 MAP kinase pathway is activated by PFT and that the p38 pathway provides an innate defense for the animal against attack by PFTs. Furthermore, the p38 pathway was subsequently shown to also protect mammalian cells against PFTs, suggesting this defense is conserved between C. elegans and mammals and expanding the known immunological functions of this key pathway. This grant proposes to follow up these results and utilize the powerful genomic and genetic tools of C. elegans to globally dissect what host genes are needed to activate the p38 MAPK pathway in response to PFT, to uncover the downstream targets of the p38 MAPK pathway that execute the defensive program, to study the role of these C. elegans p38 defense pathway genes in the defense of mammalian cells against PFT, and to study the physiological mechanisms by which the p38 pathway defends animal cells against PFT. Understanding how the p38 pathway controls defense against PFTs is likely to suggest new therapeutic strategies for dealing with a range of important bacterial pathogens that use PFTs as key virulence factors.